1. Field of the Invention
The present invention relates to a plasma processing apparatus that can be used as a deposited film forming apparatus for forming a deposited film on a substrate and to a plasma processing method that can be applied to a deposited film forming method. More particularly, the invention relates to a plasma processing apparatus used in forming a functional film, particularly a deposited film suitably used in semiconductor devices, photosensitive members for electrophotography, sensors for input of image, photographing devices, photovoltaic devices, and so on, and to a plasma processing method that can be applied to the formation of such a deposited film.
2. Related Background Art
There are many conventional methods including vacuum evaporation, sputtering, ion plating, thermal CVD, photo CVD, plasma CVD, and so on as methods for forming deposited films used in the semiconductor devices, photosensitive members for electrophotography, line sensors for input of image, photographing devices, photovoltaic devices, other various electronic devices, and optical elements, and apparatus therefor are also in practical use.
Among them the plasma CVD, which is a method for decomposing a source gas by a dc or high frequency or microwave glow discharge to form a thin deposited film on a substrate Plasma CVD is now in practical use as a favorable method for forming a deposited film of hydrogenated amorphous silicon (hereinafter referred to as "a-Si:H") used in photosensitive members for electrophotography or the like, and a variety of apparatuses therefor have been proposed heretofore.
The outline of the deposited film forming apparatus and forming method of this type will be described below.
FIG. 1 is a schematic, structural view to show an example of the deposited film forming apparatus by the RF plasma CVD process (hereinafter abbreviated as "RF-PCVD") using the frequency in the RF band as a power source, specifically, it is an example of an apparatus for forming a light receiving member for electrophotography. The structure of the forming apparatus shown in FIG. 1 is as follows.
This apparatus is principally composed of a deposition device 2100, a source gas supply device 2200, and an evacuation device (not illustrated) for depressurizing the inside of a reaction vessel 2101. Inside the reaction vessel 2101 in the deposition device 2100 there are a cylindrical substrate 2112, a substrate support 2113 internally provided with a heater for heating the substrate, and source gas inlet pipes 2114, and a high frequency matching box 2115 is connected to a cathode electrode 2111 composing a part of the reaction vessel 2101. The cathode electrode 2111 is insulated from the ground potential by insulators 2120 and a high frequency voltage can be applied between the cathode electrode 2111 and the cylindrical substrate 2112 also serving as an anode electrode while being maintained at the ground potential through the substrate support 2113.
The source gas supply device 2200 is composed of cylinders 2221 to 2226 of source gases such as SiH.sub.4, GeH.sub.4, H.sub.2, CH.sub.4, B.sub.2 H.sub.6, PH.sub.3, etc., valves 2231 to 2236, 2241 to 2246, 2251 to 2256, and mass flow controllers 2211 to 2216, each source gas cylinder being connected through a valve 2260 to the gas inlet pipes 2114 in the reaction vessel 2101.
Formation of a deposited film using this apparatus can be carried out as follows using a cylindrical substrate such as a photosensitive member for electrophotography.
First, the cylindrical substrate 2112 is set in the reaction vessel 2101 and the inside of the reaction vessel 2101 is evacuated by an unrepresented evacuation device (for example, a vacuum pump). In the subsequent step, the temperature of the cylindrical substrate 2112 is controlled to a predetermined temperature of 200.degree. C. to 350.degree. C. by the heater for heating the substrate provided in the substrate support 2113.
For allowing the source gas for formation of a deposited film to flow into the reaction vessel 2101, the following operations are carried out; after checking that the valves 2231 to 2236 of the gas cylinders and a leak valve 2117 of the reaction vessel are closed and further that the inflow valves 2241 to 2246, outflow valves 2251 to 2256, and auxiliary valve 2260 are opened, a main valve 2118 is first opened to evacuate the inside of the reaction vessel 2111 and a gas pipe 2116.
When the reading of a vacuum gage 2119 reaches about 7.times.10.sup.-4 Pa, the auxiliary valve 2260 and outflow valves 2251 to 2256 are closed.
Then the valves 2231 to 2236 are opened to introduce the gases from the gas cylinders 2221 to 2226 and the pressure of each gas is adjusted to 2 kg/cm.sup.2 by pressure adjuster 2261 to 2266. Then the inflow valves 2241 to 2246 are gradually opened to introduce each gas into the associated mass flow controller 2211 to 2216.
After completion of the preparation for film formation as described above, formation of each layer is carried out according to the following procedures.
When the cylindrical substrate 2112 reaches a desired temperature, necessary valves out of the outflow valves 2251 to 2256, and the auxiliary valve 2260 are gradually opened to introduce desired gases from the gas cylinders 2221 to 2226 through the gas inlet pipes 2114 into the reaction vessel 2101. Then the flow rate of each source gas is adjusted to a predetermined value by the mass flow controller 2211 to 2216. On that occasion the aperture of the main valve 2118 is controlled while checking the vacuum gauge 2119 so that the pressure in the vacuum vessel 2101 becomes a predetermined value. After the internal pressure becomes stable, an RF power source (not illustrated) of the frequency 13.56 MHz is set to a desired power and the RF power is guided through the high frequency matching box 2115 and cathode 2111 into the reaction vessel 2101, thus inducing a glow discharge with the cylindrical substrate 2112 acting as an anode. This discharge energy decomposes the source gases introduced into the reaction vessel and a desired deposited film comprising silicon as a main component is formed on the cylindrical substrate 2112. After the deposited film is formed in a desired thickness, the supply of RF power is stopped and the outflow valves are closed to stop the flow of the gases into the reaction vessel, thus terminating the formation of the deposited film.
By repetitively carrying out the operation similar to the above several times, a light receiving layer can be formed in a desired multilayer structure.
It is a matter of course that all the other outflow valves than those for necessary gases are closed during formation of each layer. In order to avoid the gas from remaining in the reaction vessel 2101 and in the pipes from the outflow valves 2251 to 2256 to the reaction vessel 2101, the operation to close the outflow valves 2251 to 2256, to open the auxiliary valve 2260, and to fully open the main valve 2118 to evacuate the inside of the system once to a high vacuum is carried out as occasion may demand.
In order to make the film formation uniform, it is also effective to rotate the cylindrical substrate 2112 at a desired rate by a driving device (not illustrated) during the layer formation.
Further, the gas species and valve operations described above are modified according to production conditions of each layer.
In addition to the deposited film forming apparatus and forming method by the RF plasma CVD process using the frequency in the above RF band as described above, the VHF plasma CVD (hereinafter abbreviated as "VHF-PCVD") process using the high frequency power in the VHF band is also drawing attention in recent years. Development of various deposited film forming apparatuses using VHF-PCVD is also active. This is because the VHF-PCVD process is expected to be able to achieve reduction of cost and enhancement of quality of products because of its high film deposition rate and capability of forming a high-quality deposited film. For example, Japanese Patent Application Laid-Open No. 6-287760 discloses an apparatus and method capable of being used in formation of an a-Si-based light receiving member for electrophotography. Development is also in progress of the deposited film forming apparatus that can simultaneously form a plurality of light receiving members for electrophotography and that has very high productivity, as shown in FIGS. 2A and 2B.
FIG. 2A is a schematic, longitudinal cross sectional view and FIG. 2B is a schematic, transverse cross sectional view taken along line 2B--2B of FIG. 2A. An exhaust pipe 2311 is integrally formed in a side face of a reaction vessel 2301 and the other end of the exhaust pipe 2311 is connected to an unrepresented evacuation device. Six cylindrical substrates 2305 on which the deposited film is to be formed are placed with their center axes being parallel to each other so as to surround the central part of the reaction vessel 2301. Each cylindrical substrate 2305 is held by a rotation shaft 2308 and is arranged to be heated by a heat-generating member 2307. When motors 2309 are actuated, the rotation shafts 2308 start rotating through a reduction gear system 2310, so that the cylindrical substrates 2305 start rotating on their center axis extending in the direction of the generating line.
A source gas is supplied from source gas supplying means 2312 through their source gas discharge ports (not illustrated) into a film-forming space 2306 surrounded by the six cylindrical substrates 2305. The VHF power is supplied from a VHF power source 2303 via a matching box 2304 and through a cathode electrode 2302 to the film-forming space 2306. On this occasion, the cylindrical substrates 2305 are maintained at the ground potential through the rotation shafts 2308 which act as anode electrodes.
Formation of deposited films using the apparatus described above is carried out according to the following procedures.
First, the cylindrical substrates 2305 are set in the reaction vessel 2301 and the inside of the reaction vessel 2301 is evacuated through the exhaust pipe 2311 by an evacuation device not illustrated. Subsequently, the cylindrical substrates 2305 are heated to and controlled at a desired temperature in the temperature range of about 200.degree. C. to 300.degree. C. by the heat generators 2307.
When the cylindrical substrates 2305 reach the desired temperature, the source gas is introduced through the source gas supplying means 2312 into the reaction vessel 2301. After checking that the flow rate of the source gas reaches a set value and that the pressure inside the reaction vessel 2301 becomes stable, a predetermined VHF power is supplied from the high frequency power source 2303 through the matching box 2304 to the cathode electrode 2302. This causes the VHF power to be introduced between the cathode electrode 2302 and the cylindrical substrates 2305 also serving as anode electrodes, thus inducing the glow discharge in the film-forming space 2306 surrounded by the cylindrical substrates 2305. The glow discharge excites and dissociates the source gas to form the deposited films on the cylindrical substrates 2305.
After the deposited films are formed in a desired thickness, the supply of the VHF power is stopped and then the supply of the source gas is also stopped, thus terminating the formation of deposited films. By repetitively carrying out the operation similar to the above several times, the light receiving layers are formed in the desired multilayer structure.
During the formation of deposited films the cylindrical substrates 2305 are rotated at a desired rate through the rotation shafts 2308 by the motors 2309, whereby the deposited films are formed throughout the entire circumference for the surfaces of the cylindrical substrates.
Further, Japanese Patent Application Laid-Open No. 8-253865 discloses the technology for simultaneously forming deposited films on a plurality of substrates by use of a plurality of electrodes and describes that the technology can enhance productivity and enhance uniformity of the characteristics of deposited films. Such an apparatus is shown in FIGS. 3A and 3B.
FIG. 3A is a schematic, longitudinal cross sectional view and FIG. 3B is a schematic, transverse cross sectional view taken along line 3B--3B of FIG. 3A. An exhaust port 2505 is integrally formed in an upper face of a reaction vessel 2500 and the other end of the exhaust pipe is connected to an evacuation device not illustrated. A plurality of cylindrical substrates 2501 on which the deposited film is to be formed are arranged in parallel to each other in the reaction vessel 2500. Each cylindrical substrate 2501 is held by a rotation shaft 2506 and is arranged to be heated by a heat generator 2507. The cylindrical substrates 2501 are rotated on their own axes of rotation of the shafts 2506 by driving means such as a motor not illustrated, as occasion may demand.
The VHF power is supplied from a high frequency power source 2503 via a matching box 2504 and then through cathode electrodes 2502 into the reaction vessel 2500. On this occasion, the cylindrical substrates 2501 are maintained at the ground potential through the shafts 2506 which act as anode electrodes.
The source gas is supplied into the reaction vessel 2500 through an unrepresented source gas supply means set in the reaction vessel 2500.
The formation of deposited films using the apparatus described above can be carried out according to procedures similar to those in the case of the deposited film forming apparatus shown in FIGS. 2A and 2B.
Relatively good deposited films can be formed by the methods and apparatus described above. Market demand for products using these deposited films is, however, becoming higher and higher and, in order to meet this demand, deposited films with higher quality are desired.
For example, in the case of the electrophotographic apparatus, there are very strong desires for an increase in copying speed, downsizing of the electrophotographic apparatus, and lowering of price. In order to satisfy these desires, it is indispensable to enhance the characteristics of the photosensitive member, specifically enhance chargeability, sensitivity, etc., and reduction of the production cost of the photosensitive member. Further, in the digital electrophotographic devices and color electrophotographic devices which are becoming quickly widespread in recent years, stronger desire than before exists for enhancement of the characteristics of the photosensitive member, including decrease in unevenness of image density, decrease of optical memory, and so on, because they are frequently used for copies of photographs, pictures, design images, etc. as well as character documents. Optimization of the deposited film forming conditions and the layer stacking structure of deposited films is sought in order to enhance the characteristics of the photosensitive member and reduction of production cost of the photosensitive member as described above, but, at the same time, improvement is also strongly demanded in the deposited film forming apparatus and deposited film forming method.
Thus, the deposited film forming apparatus and deposited film forming methods described previously are still susceptible to enhancement of the characteristics of deposited films and in reduction of the deposited film forming cost.
Specifically, examples of such improvement include decrease of substrate-processing time by increase of film deposition rate, increase of the number of simultaneously processable substrates, and so on. These will considerably contribute to enhancing productivity and reducing the production cost, particularly in the case where the deposited films formed are thick, as in the case of forming an electrophotographic, photosensitive member.
Further, the apparatus and methods described previously are also susceptible to improvement in uniformity and reproducibility of the characteristics of deposited films formed. Inadequate uniformity and reproducibility of the characteristics of deposited films will result in variation in the characteristics of deposited films and in turn degrade the quality of products and lower the percentage of non-defective products. Particularly, in the case of forming a member of a stacked structure of a plurality of deposited films, if this characteristic variation degrades the film characteristics of a certain layer, matching with the other layers will also be degraded and thus the entire member will be greatly affected. Further, in the case of a large-area member such as the electrophotographic, photosensitive member, even if degradation of the film quality occurs locally, removing only that portion is not possible, and thus the influence thereof will be considerable. As described above, enhancing uniformity and reproducibility of the characteristics of deposited films and suppressing variation in the characteristics of deposited films will greatly contribute to enhancing overall characteristics of the deposited films and to the reduction of the deposited film forming cost.
As described above, a deposited film forming apparatus and deposited film forming method capable of improving film deposition rate, increasing the number of simultaneously processable substrates, and enhancing uniformity and reproducibility of the characteristics of deposited films, will make it possible to achieve enhanced quality of products and the reduction of production cost. These advances are necessary and indispensable for meeting the current market demand.
Incidentally, such high-speed processing, enhanced uniformity and reproducibility, and decreased of production cost are also demanded not only for the formation of deposited films using a plasma, but also for etching, surface modification, or the like using a plasma.